This invention relates to a numerical control system of a machine tool and, more particularly, to a control instruction value generating system for generating a control instruction value expressed by a complicated function equation.
When numerically controlling a tool of a machine tool by simultaneously controlling the position of the tool on the X and Y axes of rectangular coordinates so as to move the tool along a predetermined locus on a table, the control instruction value given to the tool or table comprises a complicated function.
For example, when cutting or grinding a pin journal of the crankshaft of an internal combustion engine to give it an outer contour of a true circle, since the pin journal is located eccentrically with respect to the main journal of the engine crankshaft, it is necessary to use an instruction value expressed by a complicated function equation.
In the numerical control system of a machine tool, there is a problem of occurrence of a working error caused by time lag of the driving system (including the load) with respect to the instruction from the control device. For example, when both X and Y axes positions are simultaneously controlled so as to move the tool along a curve on a plane, the phase lag caused by the inherent load characteristics of the servo-system of the machine tool, that is the resultant of the movements along X and Y axes, has an influence upon the machining accuracy. Moreover, the effect caused by the characteristics differs depending upon the rectangular coordinates (xi, yi) of a working point. For this reason, according to the prior art system it is difficult to work at high accuracies and it is necessary to use post machining. For example, for the purpose of cutting or grinding of pin journal located eccentrically with respect to the main journal of the crankshaft of an internal combustion engine to give it a true circular cross-section, when the axis of the pin journal is revolved about the axis of the main journal by holding it by a chuck and when a control system is used to cause a tool which rotates in contact with the periphery of the pin journal to linearly reciprocate to follow said revolving motion, the time lag of a servo-system which causes linear motion of the tool head and the movement of the contact point (working point) between the workpiece and the tool are related in a complicated manner thus causing a working error.